Conventionally, automobile body sheets were obtained mainly by using cold rolled steel sheets in the past. Recently, however, rolled aluminum alloy sheets have come to be frequently used as a result of wide recognition of the importance of reductions in the weight of vehicle bodies, in response to the demand for reductions in the quantity of CO2 emission from the viewpoint of suppressing global warming. Meanwhile, rolled aluminum alloy sheets are generally inferior to cold rolled steel sheets in formability, which hampers wider use thereof. In order to enhance the formability of the rolled aluminum alloy sheets, an improvement in the formability of the blank material itself and ingenious contrivances in the method of forming the blank material are keenly demanded.
Besides, in such a kind of use, the rolled sheets are normally subjected to baking of coatings thereon, prior to use thereof. Therefore, the rolled sheets are required of a property for promising high strength after the baking (bake hardenability, or BH performance).
JP-A 4-351229 and 2006-205244 propose application of a warm deep drawing method for enhancing the formability of aluminum alloy sheets. The warm forming method does make it possible to enhance the deep drawability of aluminum alloy sheets, but application of the method to large-scale industrial production involves some problems.
Specifically, the warm deep drawing method is characterized by the need to perform deep drawing in the condition where heating of a flange part and cooling of a punch-corresponding part are being conducted. This leads to the following problems:    1. The press must be provided with functions for heating and cooling the aluminum alloy sheet, so that a longer total forming time is needed as compared with the case of cold press forming, leading to a lowered production efficiency and an increased forming cost.    2. Since forming is conducted in a warm condition, an ordinary lubricant for cold forming cannot be used, and, therefore, development of a novel lubricant is needed.    3. The press is complicated in configuration, resulting in a raised equipment cost.    4. As the press is complicated more, there arises uneasiness about quality control.
Meanwhile, the warm deep drawing method is a method wherein that part of an aluminum alloy sheet blank to be formed at which the extent of working will be large is locally heated and softened, prior to the forming. Paying attention to the moment of forming, therefore, the warm deep drawing method can be said to be a method in which enhanced formability is contrived by locally imparting a strength difference to the aluminum alloy sheet blank. In this connection, as other methods for similarly contriving enhanced formability by providing a strength difference to the aluminum alloy sheet blank, a method in which the blank is preliminarily subjected to a local heat treatment has been known (refer to, for example, JP-A 2000-117338 (hereinafter referred to Patent Document 3)). This method is considered to be particularly effective when applied to age-hardenable alloys in which a large change in strength is obtainable through solutionizing and precipitation in the matrix by a heat treatment, such as the Al—Mg—Si based alloy used mainly for automobile body sheets.
Here, in the technology disclosed in Patent Document 3, the strength difference is induced in the alloy sheet blank by utilizing the fact that, during when the Al—Mg—Si based alloy sheet to be shipped after a solution treatment at an aluminum rolling maker is held at room temperature, extremely fine precipitates composed of Mg and Si are formed evenly and finely in the matrix due to normal-temperature aging, whereby the strength is enhanced as compared with the strength immediately upon the solution treatment. Specifically, in the technology according to Patent Document 3, it is described that a local strength difference can be imparted to the aluminum alloy sheet by a treatment carried out comparatively inexpensively and in a short time, through utilizing the fact that the above-mentioned precipitates formed at room temperature are easily re-dissolved by heating to a comparatively low temperature of 250° C. or above for a short time, whereby the strength at the heated part is lowered.
Meanwhile, in the technology disclosed in Patent Document 3, the formability of an aluminum alloy sheet blank is enhanced on the premise that the blank is press formed in the condition where the periphery thereof is perfectly fixed by clamping; thus, that region in the blank surface which underlies and is to be contacted by the punch at the time of press forming, exclusive of the region to be contacted by a shoulder part of the punch, is softened by heating so as to contrive enhanced formability. In this case, however, a problem has been found in that strain is concentrated in the region underlying the punch and being softened, and the sheet thickness is considerably lowered locally in this region, leading to a lowered rigidity of the formed product. In addition, since the press forming is conducted in the condition where the periphery of the blank is perfectly fixed, inflow of material from the peripheral held-down part of the blank is not permitted at all, so that the extent of enhancement of formability is limited. Further, in the case of an automobile body sheet being in consideration, bending at a peripheral part of the formed product (hemming) is often conducted after press forming. In this connection, in the technology of Patent Document 3, the sheet region underlying the punch, namely, a central part of the sheet is heated, whereas the peripheral part of the sheet is left in the state upon age precipitation due to normal-temperature aging, and bendability is very poor in this peripheral part, leading to cracking in the bent part.